Rubicon is a noun that means ‘an action that once done cannot be reversed.’

Our word of the day was named after a river in Ancient Rome that was used as a border. At one point it was decreed by Julius Caesar that crossing the Rubicon was a violation of the law.

We now use the word to describe an act that once done, cannot be undone — just like crossing the Rubicon. For example: When Tommy deleted his profile from his favorite social media site, he instantly regretted it, because he knew he could never put it back up. Right then and there, he knew he was crossing a rubicon.